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https://issues.apache.org/jira/browse/HDFS-14617?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16874945#comment-16874945
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Stephen O'Donnell commented on HDFS-14617:
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The technique I used here is very similar to what was used in HDFS-7784. It
also used sections written to the image index to allow the work to be split
easily.
I had a quick look at HDFS-13700 before and the code seems more complex to me,
but I did not look at it in too much detail. One of my goals is to allow the
image to be loaded in parallel, while maintaining code which is easy to follow
and understand. I am tidying up some of my current code today and then I should
be able to post the version 1 patch.
I am not sure what circumstances we need to increase the fsimage version - I
would need some guidance on that. In the changes I have done, I have just added
more entries to the existing file index. There are no changes to the protobuf
definitions and I believe the existing loader would load this image and just
ignore the extra index entries. Therefore this new image should be backward
compatible with earlier versions and hence we may not need to increase the
image version.
> Improve fsimage load time by writing sub-sections to the fsimage index
> ----------------------------------------------------------------------
>
> Key: HDFS-14617
> URL: https://issues.apache.org/jira/browse/HDFS-14617
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
>
> Loading an fsimage is basically a single threaded process. The current
> fsimage is written out in sections, eg iNode, iNode_Directory, Snapshots,
> Snapshot_Diff etc. Then at the end of the file, an index is written that
> contains the offset and length of each section. The image loader code uses
> this index to initialize an input stream to read and process each section. It
> is important that one section is fully loaded before another is started, as
> the next section depends on the results of the previous one.
> What I would like to propose is the following:
> 1. When writing the image, we can optionally output sub_sections to the
> index. That way, a given section would effectively be split into several
> sections, eg:
> {code:java}
> inode_section offset 10 length 1000
> inode_sub_section offset 10 length 500
> inode_sub_section offset 510 length 500
>
> inode_dir_section offset 1010 length 1000
> inode_dir_sub_section offset 1010 length 500
> inode_dir_sub_section offset 1010 length 500
> {code}
> Here you can see we still have the original section index, but then we also
> have sub-section entries that cover the entire section. Then a processor can
> either read the full section in serial, or read each sub-section in parallel.
> 2. In the Image Writer code, we should set a target number of sub-sections,
> and then based on the total inodes in memory, it will create that many
> sub-sections per major image section. I think the only sections worth doing
> this for are inode, inode_reference, inode_dir and snapshot_diff. All others
> tend to be fairly small in practice.
> 3. If there are under some threshold of inodes (eg 10M) then don't bother
> with the sub-sections as a serial load only takes a few seconds at that scale.
> 4. The image loading code can then have a switch to enable 'parallel loading'
> and a 'number of threads' where it uses the sub-sections, or if not enabled
> falls back to the existing logic to read the entire section in serial.
> Working with a large image of 316M inodes and 35GB on disk, I have a proof of
> concept of this change working, allowing just inode and inode_dir to be
> loaded in parallel, but I believe inode_reference and snapshot_diff can be
> make parallel with the same technique.
> Some benchmarks I have are as follows:
> {code:java}
> Threads 1 2 3 4
> --------------------------------
> inodes 448 290 226 189
> inode_dir 326 211 170 161
> Total 927 651 535 488 (MD5 calculation about 100 seconds)
> {code}
> The above table shows the time in seconds to load the inode section and the
> inode_directory section, and then the total load time of the image.
> With 4 threads using the above technique, we are able to better than half the
> load time of the two sections. With the patch in HDFS-13694 it would take a
> further 100 seconds off the run time, going from 927 seconds to 388, which is
> a significant improvement. Adding more threads beyond 4 has diminishing
> returns as there are some synchronized points in the loading code to protect
> the in memory structures.
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